In many sectors of industrial process and instrumentation engineering, precise knowledge of a given physical variable is required for optimum process control. In this case, such a sensor arrangement is required to meet high standards such as robust construction, high resistance to vibration and thermal shock, insensitivity to chemically corrosive media and, if applicable, to radioactive radiation. These conditions must be satisfied cumulatively depending on the place of installation. Furthermore, the sensors should also exhibit high measurement accuracy and high reproducibility over long periods of time.
It is known that the characteristic of a measuring sensor changes as a result of events during use and aging. As the deviation from the known characteristic of the measuring sensor increases, its measurement accuracy falls. High measurement accuracy can hence only be guaranteed if the accuracy of the sensors is checked from time to time by calibration. The sensor has to be removed from the installation for this purpose.
EP 1182 438 B1 discloses a sensor arrangement comprising two different sensor types, in which a correction value for the measured value of the physical variable is calculated during an initialization phase, in which the temperature range of interest is traversed, from the difference in the detected measured values of the sensor of the first type and of the sensor of the second type, and is added so that the calculated resultant measured values for both sensors are practically equal. In the subsequent measurement and operation phase, both the respective resultant measured values and their deviations are monitored in order to detect aging-dependent drift phenomena, and an alarm is given if a limit is violated.
Owing to the individual sensors having a different response to the dynamic behavior of the process, during the initialization phase there may be a dynamic mis-calibration of the sensor arrangement resulting later in a false alarm.
In addition, there can be an accumulation of effects caused by different measurement principles, by the distance of each individual sensor from the source of the physical variable, hysteresis and manufacturing tolerances amongst identical types of sensors.
Furthermore, the initialization phase of the known method is linked to a semi-continuous temperature profile which regularly involves intervention in the process.